What is DT? is it HD?

I am setting up my favorite channels and i notice, for instance CBS is listed WCBSDT. Why is it not WCBSHD?

All my Network channels are DT.. With stations like USA, TNT, FX are HD?



Now i know all HD tv is DT, but not all DT is HD... But why is there a difference in the call letters (DT/HD?



Am i loosing quality with the DT channels?

Thanks